1. Field of the Invention
The present invention relates to a surface-mounted thermal-sensitive resistance apparatus, such as a positive-temperature-coefficient (PTC) or negative-temperature-coefficient (NTC) element, which is mainly applied to a printed circuit board (PCB) for sensing overloading current and abnormal temperature variation to protect elements on the PCB.
2. Description of the Related Art
A conductive composition material made of an organic polymer, such as polyethylene, and a conductive material, such as carbon black, metal particle or metal powder, has many applications. Among the applications, the resistance variation of a non-linear PTC apparatus is the most outstanding. The characteristic of the PTC apparatuses can be used to design electronic components or electrical apparatuses for protecting circuits from damage by overheating or over-threshold current.
In other words, the resistance of the PTC material is very low at normal temperatures and increases immediately by a magnitude of at least 103 when the temperature is over a threshold or a surge of electrical large current flows through the PTC material. The resistance-varying feature will efficiently suppress or reduce an over-current to protect a circuit from burning off. Another advantage of the PTC apparatus is that when the temperature returns to normal or the over-current disappears, the resistance decreases to its original state of operation again. Unlike the ordinary fuses, which could only trip once and have to be replaced after trip, the PTC apparatus could trip many times and could easily be reset. For the advantage of the resettable characteristics of the PTC apparatus, the PTC apparatus is largely used in high-density electronic circuits.
Nevertheless, it is a trend in the modern high-density circuit design that all components on the circuit board must be small, thin and light and able to be mounted on a surface of the circuit board. Therefore, the PTC apparatus made of conductive polymer material with a pair of conductive metal electrode foils has been designed in a lot of surface-mounted components. For example, U.S. Pat. No. 5,089,801 shows a PTC apparatus and a pair of conductive metal terminals adhered to the conductive electrode on upper and lower planes of PTC apparatus. In a process of manufacturing the prior art PTC apparatus, the stability of adhesion between conductive metal terminal and the planar conductive electrodes on upper and lower planes of the PTC apparatus is a major shortcoming. The disadvantage of the prior art is that the connection between the PTC apparatus and the pair of conductive metal terminals is not strong enough to withstand a reflow process, and usually a disconnection occurs.
U.S. Pat. No. 5,864,281 shows a method in which conductive electrode of metal foils on both surfaces of a PTC apparatus are etched based on a designed pattern until a PTC layer appears. Then, along etched trenches, the PTC apparatus is broken into pieces by external force to form resistance component. This patent emphasizes that the PTC apparatus is made by the manner of breaking. As a result, fractured edges are found in the PTC apparatus. This patent did not teach about how to resolve a thermal conduction problem by changing the structure of the PTC apparatus.
U.S. Pat. Nos. 5,831,510 and 5,852,397 disclose other designs of a PTC apparatus, wherein the design of a pair of electrodes is the same as an ordinary printed circuit board in which an upper electrode is connected to a lower electrode via electroplated with a conductive material. Although the design of these patents overcome the disadvantage disclosed in U.S. Pat. No. 5,089,801 by eliminating a pair of planar conductive metal terminals, the following problems should be overcome:
(1) The apparatus exhibits high heat transfer rate, which causes the resistance of the apparatus to be strongly affected by its environments, such as line width, line thickness and position on the printed circuit board. Therefore, the magnitude of resistance variation from normal (xe2x80x9cuntrippedxe2x80x9d) state to tripped state becomes unpredictable and hard to control.
(2) Due to the difference of thermal expansion coefficients between a metal electrode and the conductive organic polymer material, the stress embedded in the PTC apparatus is not uniform. Consequently, the metal electrode is susceptible to delamination during service and thereby loses its electrical function.
(3) Each of the electrode foils on the surfaces of the upper and lower end should be separated by etching to form two sections on each of the surfaces. As a consequence, effective area is reduced.
Accordingly, the objective of the present invention is to resolve the drawbacks of prior arts and to provide with a novel thermal-sensitive resistance apparatus for surface mounting.
In order to achieve the above-mentioned objective, the present invention introduced a novel design and process. The present invention proposes an electrical apparatus for surface mounting, which allocates the electrode films of both planar sides of a normal thermal-sensitive resistance apparatus, such as a PTC apparatus to laminate with an outer electrode layer, and a plurality of vias electroplated with conductive material are used to connect to any plane. It is convenient to mount the apparatus according to the present invention on a printed circuit board.
The present apparatus mainly comprises:
(1) a thin-board resistive component, which (a) is made of a polymer material in which conductive particles are distributed, (b) has a characteristic of positive or negative temperature coefficient, (c) has opposite first and second surfaces, and (d) has opposite first and second end surfaces, respectively extending from the first surface to the second surface;
(2) a first conductive component, disposed on the first surface of said thin-board resistive component and extending to the first end surface;
(3) a second conductive component, disposed on the second surface of said thin-board resistive component and extending to the second end surface;
(4) a first insulating film, disposed on said first conductive component and extending to the second end surface of said thin-board resistive component;
(5) a second insulating film, disposed on said second conductive component and extending to the first end surface of said thin-board resistive component;
(6) a first electrode, including a main body and an interconnection via, wherein said main body includes a pair of metal foils disposed on said first and second insulating films, and said interconnection via includes a conductive film connected to said metal foils and to said first conductive component along the first end surface; and
(7) a second electrode, including a main body and an interconnection via, wherein said main body includes a pair of metal foils disposed on said first and second insulating films, and said interconnection via includes a conductive film connected to said pair of metal foils and to said second conductive components along the second end surface.
As well, on the first and second insulating film, (8) a pair of isolating components, such as a solder mask can be further included, which are situated respectively on said first and second insulating films for isolating the first and second electrodes.
Furthermore, the present invention also comprises a multi-layer resistance apparatus, which folds at least two layers of the conductive resistance apparatus, comprising:
(A) a first resistive assembly, including: (1) thin-board resistive components, each of said components has: (a) polymer materials in which conductive particles are distributed; (b) a characteristic of positive or negative temperature coefficient; (c) opposite first and second surfaces; (d) opposite first and second end surfaces, respectively extending from the first surface to the second surface; (2) a first conductive component, disposed on the first surface of said thin-board resistive component and extending to the first end surface; (3) a second conductive component, disposed on the second surface of said thin-board resistive components and extending to the second end surface; (4) a first insulating film, disposed on said first conductive component and extending to the second end surface of said thin-board resistance component; (5) a second insulating film disposed on the second conductive component and extending to the first end surface of said thin-board resistive component.
(B) at least one second resistive assembly of the same structure as said first resistive assembly and superimposed on said first resistive assembly;
(C) a first electrode, including a main body and an interconnection via, wherein said main body includes a pair of metal foils disposed on said first and second resistive assemblies and said interconnection via includes a conductive film connected to the pair of metal foils and to said first conductive components along the first end surface; and
(D) a second electrode, including a main body and an interconnection via, wherein said main body includes a pair of metal foils disposed on said first and second resistive assemblies; said interconnection via includes a conductive film connected to the pair of metal foils and to said second conductive components along the second end surface.
Besides, on the first and second resistance assembly, (E) a pair of isolating components, such as a solder mask can be further included, which are situated on the first and second resistance assembly for isolating the first and second electrode.
Different from prior art, the present invention teaches a novel way to construct at least a two-layer resistive apparatus by adhesion, and to eliminate the prior design shortfall that the thermal diffusion of the surface mounting resistance is sensitive to on line width on the circuit board and environments. Additional disadvantages, such as non-uniformity of stress, easy bending of PTC resistive assembly, and variation of resistance due to the difference of thermal expansion coefficients between metal foil electrodes and the conductive polymer material, can be overcome by the designing device with better dimensional stability in the X-Y axis according to the present invention. In prior designs, such as U.S. Pat. Nos. 5,831,510 and 5,852,397, each side of the electrodes must have an etching line, which results in the reduction of the effective area. The present invention only etches small area of first and second conductive components surrounding the second and first end surfaces respectively. This design could increase the effective area and reduce device impedance. The final resistance of the multi-layer apparatus formed by connecting multiple layers in parallel could be reduced, and is equal to the reciprocal of the sum of 1/R1, 1/R2, 1/R3, . . . , 1/Rn, wherein R1 to Rn denotes the resistance of resistive assembly layer 1 to resistive assembly layer n respectively.